This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving servo writing operations such as servo track write verification by biasing multiple magneto-resistive read elements of a disc drive.
Hard disc drives are used in modern computer systems to enable users to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive is generally composed of a head/disc assembly (HDA) which houses requisite mechanical portions of the drive and a printed wiring assembly (PWA) which supports requisite electronic portions of the drive.
The HDA includes a base deck to which various components are mounted and a top cover which cooperates with the base deck to form a sealed housing to reduce particulate contamination. Within the housing, a disc stack is formed from one or more magnetic recording discs which are axially aligned for rotation by a spindle motor at a constant, high speed, such as 10,000 revolutions per minute during normal disc drive operation.
A rotary actuator assembly is mounted adjacent the disc stack and includes a plurality of rigid arms which extend into the stack between adjacent discs, as well as above and below the top and bottom discs. The rigid arms support flexible suspension assemblies which in turn, support a corresponding number of read/write heads adjacent the surfaces of the discs. One head is provided for each disc surface to read data from and to write data to the corresponding surface.
Of particular interest are magneto-resistive (MR) heads, which utilize thinfilm inductive write elements to write data and MR read elements to read previously written data. A typical MR read element is formed from an alloy of materials so as to have a baseline electrical resistance which varies in the presence of a magnetic field of a selected orientation. By passing a bias current through the MR element, the selective magnetization of a corresponding track can be determined in relation to variations in voltage detected across the MR element.
A preamplifier/driver circuit is typically mounted to the side of the actuator assembly and includes a write driver circuit to apply write currents to a selected write element during a write operation. The preamplifier/driver circuit further includes a bias current source which is sequentially connected to the appropriate read element to effect a read operation.
The electronics provided on the disc drive PWA primarily serve to control the operation of the HDA and to communicate with a host computer in which the disc drive is mounted. Generally, the top level functional blocks on the PWA include a read/write channel which controls the reading and writing of data from and to the discs, a spindle motor control circuit which controls the rotation of the spindle motor, and a servo control circuit which controls the position of the heads.
Aspects of a typical servo control circuit are discussed in U.S. Pat. No. 5,262,907 issued to Duffy et al., assigned to the assignee of the present invention. The servo control circuit positions the heads relative to the tracks through the application of current to a coil of a voice coil motor (VCM) within the HDA, the coil being mounted to the actuator opposite the heads. The tracks are defined from servo data written to servo data fields on the surfaces of the discs during the manufacturing of the HDA. The servo data are stored as a series of radially extending servo wedges on each of the disc surfaces, with the servo wedges composed of adjacently aligned servo data fields, with each servo data field in each wedge defining a unique track. Hence, by periodically transducing the servo data associated with a particular track, the servo control circuit can adjust the current applied to the coil to adjust the position of the corresponding head. User data fields, which are used to store user data from the host computer, are subsequently defined between adjacent servo fields during a disc drive formatting operation.
Conventionally, the servo data are written using a servo track writing system, also commonly referred to as a servo track writer (STW). A typical servo track writer comprises a fixture on which each HDA in turn is mounted. Once mounted, the servo track writer proceeds to write the servo data using the heads of the HDA. Thus, a typical servo track writer includes control circuitry which generally emulates portions of the electronics disposed on the PWA, as well as a closed loop positioning system which both detects the radial position of the heads and mechanically advances the heads. Access to the actuator is achieved by providing an opening in the base deck of the HDA which is later sealed.
Position detection and feedback are usually carried out using a laser inferometer or other precise optical displacement instrumentation. Mechanical advancement of the heads can be carried out by inserting a pusher pin assembly into the HDA to engage and move the actuator assembly. More recently, positioning systems have also been developed which apply current to the actuator coil, thereby utilizing the VCM to advance the position of the heads. The control and positioning systems are usually interfaced with a personal computer (PC) which provides a graphical user interface for the STW operator to control the operation of the system.
The writing of servo data is an important, but correspondingly time consuming, portion of the HDA manufacturing process. A typical STW operation can take up to several hours per HDA, so that disc drive manufacturers will often implement large numbers of STW stations to accommodate large scale disc drive production efforts. For purposes of efficiency, a servo track writer does not typically write all of the servo data on one disc surface before moving to the next surface; rather, each of the heads are selected in turn so that, after a selected head writes a portion of the data at a given radius, the next head is selected and used to write a corresponding portion of the servo data at a selected angular distance from the first portion, and so on. This results in the servo data on each disc surface being offset with respect to the data on adjacent surfaces, resembling steps of a spiral staircase. This is repeated multiple times around the circumference of the discs, as disc drives typically have from 30 to 90 servo wedges on each surface.
Once all of the servo data have been written at a given radius, the servo track writer proceeds to verify the accuracy of the writing operation. This generally entails sequentially applying a read bias current to each of the heads in turn to transduce and verify the servo data from the respective surfaces in the order in which the data were initially written to the surfaces. Because only one read bias current source is available in the preamplifier circuit, the elapsed time between successive servo data fields on successively read data surfaces must be long enough to allow the current to be switched from the first head to the second head and to allow the second head to reach an equilibrium state before the second servo data field is read. Hence, it may require a reduction in the rotational speed of the disc, or additional revolutions of the discs, in order to enable the servo track writer to verify all of the servo data. This can present a bottleneck in a disc drive manufacturing process, requiring additional investments in resources to meet the required process throughput.
Accordingly, there is a continued need for improvements in the manner in which data are tranduced from disc recording surfaces, such as during the read verification operations carried out during the writing of servo data. It is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for providing improved read operations upon disc recording surfaces through the simultaneous biasing of multiple magneto-resistive servo writing.
In accordance with preferred embodiments, a disc drive includes a head/disc assembly (HDA) housing a plurality of disc recording surfaces and an actuator assembly which supports a corresponding plurality of heads adjacent the surfaces. A preamplifier circuit is mounted to the actuator assembly and includes a first read bias current source which generates a first read bias current of selected magnitude. The preamplifier circuit further includes a second read bias current source which generates a second read bias current of selected magnitude independently of the first read bias current. A head selection circuit of the preamplifier circuit selects first and second heads of the HDA so that the first and second read bias currents are simultaneously directed to the first and second heads, respectively.
In this way, a servo writing operation, such as a verification operation used to verify accuracy of servo data written to the disc recording surfaces during a servo track write operation, can be carried out efficiently by sequentially applying read bias currents to successive pairs of the heads. The read bias current applied to a selected one of each pair of the heads is used to transduce the data from the associated disc recording surface, while the read bias current applied to the remaining one of each pair of the heads is used to prepare the remaining one of each pair of the heads to subsequently transduce the data from the associated disc recording surface.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.